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Employee benefit plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Components of net periodic pension cost:    
Salaries and employee benefits expense $ 6,570 $ 7,856
Pension Benefits    
Components of net periodic pension cost:    
Salaries and employee benefits expense 306  
Interest cost 105 54
Expected return on plan assets (322) (123)
Amortization of unrecognized net gain 76 22
Net periodic benefit $ (141) $ (47)